1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas and, more particularly, to a catalyst for purifying an exhaust gas to be discharged from an internal combustion engine for a vehicle.
2. Related Art
An exhaust gas purifying catalyst of this kind, as known in the prior art, is constructed by carrying a catalytic element such as platinum, palladium or rhodium on a carrier such as alumina. This exhaust gas purifying catalyst exhibits a high purifying ability because its catalytic particles keep a predetermined particle diameter and disperse homogeneously to some extent. However, this exhaust gas purifying catalyst is used in an environment of a hot atmosphere as high as several hundreds .degree.C. In addition, an engine for a lean-burn run at a large air/fuel ratio has been developed in recent years so that the exhaust gas purifying catalyst is held in an environment of a high oxygen concentration. As a result, the sintering of the catalytic particles gradually advances to raise a disadvantage that the catalyst is seriously deteriorated.
In the prior art, there have been investigated a variety of techniques for inhibiting the agglomeration of catalyst particles, including the catalyst which is constructed by dispersing and adhering magnesium to the catalytic activation particles of platinum or palladium, as disclosed in Japanese Patent Laid-Open No. 186347/1991 (JP-A-3-186347). In this catalyst, it is thought that the catalytic activation particles of palladium or the like are prevented from agglomeration by solidifying magnesium thereon.
In Japanese Patent Laid-Open No. 131830/1996 (P-A-8-131830), on the other hand, there is disclosed an exhaust gas purifying catalyst which is constructed with platinum and alumina carrying the former and is coated in its entirety with ceria (CeO.sub.2). In the catalyst disclosed, the oxidation of platinum and its migration in gas phase are prevented by the oxygen occluding action of ceria so that the sintering of platinum is prevented.
In Japanese Patent Laid-Open No. 246343/1995 (JP-A-7-246343), moreover, there is disclosed a process for producing a catalyst by dissolving metal alkoxide gradually in the presence of water into a dispersion of super fine particles having catalytic activity to cause a hydrolysis of the metal alkoxide thereby to produce a colloidal metal oxide. This catalyst is exemplified by silica gel of about 20 to 40 nm (nanometers) carrying platinum or rhodium particles of 4 nm.
Here, the sintering of noble metal particles to be used as the catalytic particles is divided into the sintering to be caused by a particle migration from a relatively low temperature and the sintering to be caused by an atomic migration at a relatively high temperature. However, the invention, as disclosed in Japanese Patent Laid-Open No. 186347/1991 (JP-A-3186347), is intended to prevent the agglomeration of the catalytic activation particles by the solid solution of magnesium but has no function to prevent or inhibit the particle migration itself. As a result, the sintering cannot always be sufficiently prevented so that the catalytic activation is liable to lower with the time.
On the other hand, the catalyst, as disclosed in Japanese Patent Laid-Open No. 131830/1996 (JP-A-8-131830), is intended to lower the peak concentration of oxygen by the oxygen occluding action of ceria thereby to prevent the migration of PtO.sub.2 in gas phase. However, the particle migration, as would occur at a lower temperature, is difficult to inhibit. After all, there is a problem that the activity is gradually lowered by the sintering of the catalytic particles of platinum or the like.
In the catalyst producing process disclosed in Japanese Patent Laid-Open No. 246343/1995 (JP-A-7-246343), moreover, the carrier is formed after the super fine particles having the catalytic activity are formed. However, it is difficult to prepare metal oxide particles having equal particle diameters because the metal alkoxide for the carrier material is dissolved into the dispersion of those super fine particles. Moreover, the resultant structure, in which the catalytically active super fine particles are adhered to the surfaces of the metal oxide particles, makes it difficult to give a function to prevent or inhibit the migration of the catalytic particles. In the catalyst thus produced by this process, therefore, the drastic inhibition of the sintering of the catalytic particles is still difficult and leaves room for improvements.